Light emitting diodes (LEDs) provide an energy-efficient light source and are increasingly being used instead of fluorescent and halogen gas lamps for high capacity lighting needs, such as street lamps. In order to increase the amount of light generated, LEDs are often incorporated into street lamps, which can lead to significant problems of overheating. The performance and lifetime of the LEDs is degraded if the operating temperature exceeds a threshold level. The useful life of an LED street lamp is sometimes specified as the number of hours of operation before which the luminous output of the lamp drops to half of its initial output. Empirical data suggests that there is an inverse exponential relationship between the useful life of an LED lamp and the amount by which the average operating temperature exceeds a threshold level. For example, some phosphors in LEDs have been found to degrade if the temperature of the phosphor exceeds 165 degrees Celsius over an extended period. Thus, dissipating the heat generated by the LEDs in the street lamp is a problem that must be solved.
The LEDs of a street lamp are enclosed by the shell of the street lamp. The shell typically has a metal upper cover and a transparent lower cover. The shell typically has openings at the top that allow the heat generated by the LEDs to escape. However, the openings at the top allow dust, moisture and insects to enter the shell, which can accumulate in the transparent lower cover and block much of the light that is generated by the LEDs.
FIG. 1 (prior art) shows an existing LED street light 10 that does not permit dust, moisture and insects to obstruct the transparent lower cover 11 through which the generated light shines onto the street. Transparent lower cover 11 is kept free of dust, moisture and insects by completely sealing off the lower compartment of the shell of the street lamp. Consequently, most of the heat generated by the LEDs must be dissipated through the upper compartment. The heat is transmitted from the LEDs to a heat conducting plate 12. The heat then passes on to heat-dissipating fins 13 via a heat guiding piece 14. Heat is dissipated out of the upper compartment by fans 15 that blow heated air out of venting slots 16 in the upper cover 17. For additional details on this prior art method of dissipating heat from a street lamp, see U.S. Pat. No. 7,278,761 to Kuan entitled “Heat Dissipating Pole Illumination Device.”
This prior art method of dissipating heat from an LED street light has multiple disadvantages. First, although dust, moisture and insects are prevented from entering the lower compartment of the street lamp shell, they will nevertheless enter the upper compartment through the venting slots 16 in the upper cover 17. The dust, moisture and insects that fall through the venting slots 16 will collect in the upper compartment and clog the fans 15 and the spaces between the fins 13, thereby reducing the ability of street lamp to dissipate heat. Second, the fans 15 have moving parts and will likely malfunction, especially if they are subjected to the dust, moisture and insects that enter through the venting slots 16. Moreover, the fans 15 also require a power supply, which might not be able to be shared with the LEDs. Finally, the fans 15 add to the cost of the street lamp.
A method is sought for dissipating heat from an LED street lamp that does not allow dust, moisture and insects to enter through venting slots in the upper cover of the street lamp and that does not require fans.